Optical communications systems transfer vast amounts of information over substantial distances using optical transmissions, typically through a fiber optic cable or similar optical medium. Transmissions through an optical medium degrade over distance in a different manner than electrical transmissions. Typically, dispersion of the optical signal is a substantial limitation on the length of the fiber optic channel before conversion to electrical signals is required for regeneration of the communicated data signal. Thus, for extreme distances, a series of transmitters and receivers (or transceivers) are linked by sections of fiber optic cable. The communications signal is converted back to electrical signals and regenerated, e.g., amplified, in electrical form.
Optical dispersion causes pulse broadening that impairs receiver performance, particularly when the transmitted optical signal is detected using square-law detection. If the pulses broaden too much, then the symbols used to encode the communications signals “overlap,” producing intersymbol-interference.
A representation of a basic optical communications system is shown in prior art FIG. 1A. An input signal X(t) 105 to be sent over the optical channel is received at a transmitter 150 and modulated onto an optical beam 155. The optical beam 155 has a frequency domain representation X(ω) which is modified by dispersion response of the channel D(ω) 170. At the output of the channel, a receiver 185 receives a channel output beam 175 (having a frequency domain representation Y(ω)=D(ω)X(ω)). The ideal receiver 185 converts the output beam into a electrical receive signal 190. If the system were unaffected by dispersion (and other noise sources), the received signal Y(t) 190 would be recognized as the transmitted signal X(t) 105.
The most common method to address dispersion impairments in fiber optic transmission is the use of dispersion compensation modules (DCM). A DCM is a specially-designed optical filter that compensates the pulse-spreading effect, but is costly, bulky, and lossy.
An example of how a DCM may be used is shown in the optical communications system in prior art FIG. 1B. Somewhere along the signal path, one or more DCMs 160 act on the optical signal X(ω) 155 with a correction function C(ω) to create a corrected signal X′(ω) 165. The channel still creates dispersion in the optical beam as represented by dispersion block D(ω) 170. The correction function C(ω) for the DCM 160 is chosen so that C(ω) cancels out as much of the channel dispersion D(ω) 170 as possible. When the signal reaches the receiver 185, the output signal Y(ω) 180 now has the frequency representation given by Y(ω)=D(ω)C(ω)X(ω). If the correction function C(ω) has been chosen correctly, then the product D(ω)C(ω) is independent of ω, making Y(t) simply an attenuated and time-shifted version of X(t). Such a compensation function can be difficult to achieve in the optical domain.